Conventional dice are small cubical blocks bearing a pattern of dots on each face to indicate a numerical value from one to six. In many popular games, two or more dice are tumbled to produce an essentially random combination of numbers. One such game, using five dice, is known as Yacht, and is also commonly referred to as "Yahtzee", a trademark of the E. S. Lowe Company, Inc.
A Yacht player scores points by throwing the dice to obtain combinations which include sequences of consecutive numerical values, or repeated occurrences of the same numerical value. These combinations correspond closely to hands in the card game of poker, and are usually described in the same terms. Thus, a sequence of consecutive values is called a "straight", while repeated occurrences are called "three of a kind", "four of a kind", and so forth, because the numerical value on the face of a die is analogous to the rank of a playing card.
The analogy to playing cards is not complete, however, because playing cards are marked with a suit as well as a rank. Since standard dice have no attribute analogous to a suit, certain significant card combinations cannot be parallel in Yacht. For example, one significant combination of cards in poker, known as a "flush", consists of any five cards of the same suit. Another, called a "straight flush", is a sequence of consecutively ranked cards of the same suit. Neither flushes nor straight flushes can be achieved in Yacht using conventional dice.
A variant of Yacht can be envisioned in which the dice bear a suit attribute, as well as a ranked numerical attribute, so that flushes and straight flushes are possible. Ideally, the variant would have three additional properties. First, the probability of obtaining a given combination, such as a flush, in any one suit, should be the same as the probability of obtaining a similar combination in any other suit. Second, the dice should be constructed so that each numerical attribute value appears only once on each die, so that the probability of obtaining ordinary straights and repeated values is unaffected by the addition of suit attribute. Third, it is desirable to keep the number of numerical attribute values, and hence, the number of sides on each die, to a minimum, so that the difficulty of obtaining any particular combination is not unduly increased.
One prior art attempt to provide such a variant employed five identical cubical dice, with each face bearing one of three colors and a numerical value. Opposite sides of a die carried the same color, and each color indicated a suit. This scheme permitted simple flushes to be produced, but being identical, the dice could not form a straight flush, since only two numerical values appeared with each color.
Other types of game dice have been designed to substitute for cards in poker, such as those disclosed in U.S. Pat. Nos. 3,608,905 of Edison, 645,112 of Mapes and 614,524 of Yardley. However, the dodecahedral and decahedral shapes suggested in those patents are not optimal for use in a Yacht variant because the large range of rank or numerical attribute values makes each particular combination unnecessarily difficult to obtain.